Inkjet printing is a non-impact method for producing printed images by the deposition of ink droplets in a pixel-by-pixel manner to an image-recording element in response to digital signals. There are various methods that can be used to control the deposition of ink droplets on the image-recording element to yield the desired printed image. In one process, known as drop-on-demand inkjet, individual droplets are projected as needed onto the image-recording element to form the desired printed image. Common methods of controlling the ejection of ink droplets in drop-on-demand printing include thermal bubble formation (thermal inkjet (TIJ) and piezoelectric transducers. In another process known as continuous inkjet (CIJ), a continuous stream of droplets is generated and expelled in an image-wise manner onto the surface of the image-recording element, while non-imaged droplets are deflected, caught, and recycled to an ink sump. Inkjet printers have found broad applications across markets ranging from desktop document and photographic-quality imaging, to commercial printing and industrial labeling.
Ink compositions containing colorants used in inkjet printers can be classified as either pigment-based, in which the colorant exists as pigment particles suspended in the ink composition, or as dye-based, in which the colorant exists as a fully solvated dye species that includes one or more dye molecules. Pigments are highly desirable since they are far more resistant to fading than dyes. However, pigment-based inks have a number of drawbacks. Great lengths are undertaken to reduce a pigment particle to a sufficiently small particle size and to provide sufficient colloidal stability to the particles. Pigment-based inks often require a lengthy milling operation to produce particles in the sub-micron range needed for most modern ink applications. If the pigment particles are too large light scattering can have a detrimental effect on optical density and gloss in the printed image.
A second drawback of pigmented inks is their durability after printing, especially under conditions where abrasive forces have been applied to the printed image. Pigment-based inks typically reside at the surface of the imaging receiver to which they are printed and this makes the printed images particularly susceptible to abrasive forces. To this extent, pigmented inks have been formulated with various polymers, dispersants, and other addenda to provide durable images that can withstand post printing physical abuse and environmental conditions.
The degree of abrasion resistance of a printed image is also a function of time after printing. At short time intervals after printing, typically from a few minutes to a few hours, the ink undergoes several complex dynamic changes. As the ink contacts the receiver, some of the components penetrate into the receiver and the droplets can simultaneously spread laterally on the receiver surface. Carrier fluids such as water and humectants are drawn into the receiver by capillary forces and the polymer binders begin to film form. At short time intervals the binder film formation is incomplete and the resulting pigment cake is particularly susceptible to abrasive forces. Typically, the more total fluid that is printed to the receiver (and hence more water) the longer it takes for the ink to dry and form a durable image. The abrasion resistance of the image is further affected by the presence of humectants, which are employed for optimal firing performance, but which are retained in the pigment cake for some period of time. Since most humectants have much lower vapor pressures than water, they are relatively slow to evaporate and can be retained in the image receiver for several hours. Humectants can have the effect of plasticizing the polymer binder and making the surface of the image tacky or softer than if no humectant was present. Once the humectants evaporate, the resulting pigment cake, consisting primarily of pigment and binders, reaches a steady state composition, and determines the long-term abrasion resistance of the printed image.
Continuous inkjet (CIJ) printers typically consist of two main components, a fluid system and one or more printheads. Ink is delivered through a supply line from a supply reservoir to a manifold that distributes the ink to a plurality of orifices, typically arranged in linear array(s), under sufficient pressure to cause ink streams to issue from the orifices of the printhead. Stimulations are applied to the printhead to cause those ink streams to form streams of spaced droplets, which are deflected into printing or non-printing paths. The non-printing droplets are returned to the supply reservoir via a droplet catcher and a return line. U.S. Pat. Nos. 4,734,711 and 5,394,177 and EP 1,013,450 describe in detail the design of a fluid system for CIJ apparatus. The more recent development of CIJ printing apparatus and printhead fabrication can be found in U.S. Pat. Nos. 6,588,888 and 6,943,037.
Ink drop uniformity requires maintaining a uniform pressure in the printhead cavity. U.S. Pat. No. 4,614,948 describes that a positive displacement pump, such as gear pump, is preferred for use as the ink supply pump. The need to limit pulsation produced by the pump is recognized in U.S. Pat. No. 4,971,527. In addition, filters are employed at appropriate locations in fluid system to remove oversized particles prior to ink entering into printhead orifices and avoid printhead clogging.
CIJ inks traditionally have been mostly aqueous dye-based inks, where issues regarding robust system runability, such as easy start up/shut down, extended printing time without crooked jet, and reduced frequency for filter changing have been minimized. In such traditional dye-based inks, no particles are observable under the microscope. Although there have been many recent advances in the art of dye-based inkjet inks, such inks still suffer from deficiencies such as low optical densities on coated glossy paper and poor light-fastness. When water is used as the carrier, such inks also generally suffer from poor water fastness and poor smear resistance. Accordingly, the advantages associated with the use of pigmented inks would also be desirable for use in continuous inkjet printing systems.
Inkjet printers further require straight, uniform droplet streams or jets for quality printing. Current ink jet printing systems include a fluid system supporting one or more printheads. Typical ink jet printheads operate by forcing fluid through a droplet generator that contains an array of orifices, forming droplets of ink. The printhead is fully supported by the fluid system. The fluid system controls different valves and pumps to perform necessary functions for the printhead to operate reliably. These functions include cleaning, startup, and shutdown. One particular function, shutdown, provides a means to stop the operation of the printhead and fluid system over an extended period of time, and allows for a restart of the operation. If ink is left in the droplet generator during a shutdown, however, the fluids can dry in and around the orifices leaving behind non-volatile components in the form of solids or gels. Upon subsequent startups, the failure to remove or re-dissolve all of this material in and around the orifices creates disturbances in the shape or direction of the emerging jets. Dried ink solidified on the jetting nozzle or orifice can thus cause jets to deflect thereby causing print defects. Dried ink is particularly a problem with inks containing polymeric components, and in particular for dried pigmented inks containing polymeric components. This problem is typically corrected by adding humectants to the inks, and by performing maintenance procedures which include purging ink channels and nozzles with a maintenance or cleaning fluid during shutdowns and/or a physical wiping or parking process for the printhead nozzles.
In continuous inkjet printing, however, it is not practical to add large amounts of humectants to the inks as is typically practiced in drop-on-demand printing. Humectants such as glycerin or other polyhydroxylated organic compounds typically do not exceed 10 weight percent of a continuous inkjet printing ink composition. At higher humectants concentrations, the ink viscosity will become too high for proper drop formation and jetting, and drying times will become too slow for the desired high printing speeds of continuous inkjet printing. Preferably, the humectant concentration is less than or equal to about 6% by weight of the ink formulation for continuous inkjet printing inks.
In continuous inkjet printing, it is further undesirable and sometimes impossible to physically contact the printhead to clean the nozzles. Wiping or parking the printhead for cleaning purposes is thus not practical. In order to enable quality printing, removal of dried ink and foreign matter from the nozzles thus typically must be achieved by chemical cleaning alone. Furthermore, the maintenance or cleaning fluids used for chemical cleaning of the nozzles must be compatible with the imaging inks used as they will become part of the ink to some level. Ideally this level should be low, but it is seldom zero. Chemical cleaning processes further will typically not remove all of the ink residues. Therefore the maintenance fluid must be compatible with small amounts of residual ink (e.g., 2-10 weight %) over periods of several hours to several weeks, and then be capable of being quickly and completely removed by the ink when the printing process is restarted.
Ota, in U.S. Pat. No. 7,686,417, discloses maintenance fluids for a drop-on demand inkjet printer employing pigment-based inks. The fluids comprise a resin solvent such as an alcohol, an ether (for example, diethylene glycol monobutyl ether), a ketone or an ester, among others; a humectant such as glycerin, diethylene glycol, propylene glycol or triethylene glycol, among others; as well as surfactants, biocides and other additives.